1. Field of the Invention
Embodiments of the present invention generally relate to the fabrication of solar cells and particularly to a device structure and method of passivating a surface of a crystalline silicon solar cell.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon (Si), which is in the form of single, polycrystalline or multi-crystalline substrates. Because the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by traditional methods, there has been an effort to reduce the cost of manufacturing solar cells that does not adversely affect the overall efficiency of the solar cell.
FIG. 1 schematically depicts a cross-sectional view of a standard silicon solar cell 100 fabricated from a crystalline silicon substrate 110. The substrate 110 includes a base region 101, an emitter region 102, a p-n junction region 103, a dielectric passivation layer 104, front electrical contact 107 and rear electrical contact 108. The p-n junction region 103 is disposed between base region 101 and emitter region 102 of the solar cell, and is the region in which electron-hole pairs are generated when solar cell 100 is illuminated by incident photons. Dielectric passivation layer 104 may act as an anti-reflective coating (ARC) layer for solar cell 100 as well as a passivation layer for the surface 105 of emitter region 102.
The efficiency of solar cell 100 may be enhanced by use of an ARC layer. When light passes from one medium to another, for example from air to glass, or from glass to silicon, some of the light may reflect off of the interface between the two media, even when the incident light is normal to the interface. The fraction of light reflected is a function of the difference in refractive index between the two media, wherein a greater difference in refractive index results in a higher fraction of light being reflected from the interface. An ARC layer disposed between the two media and having a refractive index whose value is between the refractive indices of the two media is known to reduce the fraction of light reflected. Hence, the presence of an ARC layer on a light-receiving surface of solar cell 100, such as dielectric passivation layer 104 on surface 105, reduces the fraction of incident radiation reflected off of solar cell 100 and which, therefore, cannot not be used to generate electrical energy.
When light falls on the solar cell, energy from the incident photons generates electron-hole pairs on both sides of p-n junction region 103. In a typical solar cell, which comprises an n-type emitter region and a p-type base region, electrons diffuse across the p-n junction to a lower energy level and holes diffuse in the opposite direction, creating a negative charge on the emitter and a corresponding positive charge build-up in the base. In an alternate configuration, which has a p-type emitter region 102 and n-type base region 101 (FIG. 1), electrons diffuse across the p-n junction to form a positive charge on the emitter and holes diffuse in the opposite direction to form a negative charge build-up in the base. In either case, when an electrical circuit is made between the emitter and the base, a current will flow and electricity is produced by solar cell 100. The efficiency at which solar cell 100 converts incident energy into electrical energy is affected by a number of factors, including the recombination rate of electrons and holes in solar cell 100 and the fraction of incident light that is reflected off of solar cell 100.
Recombination occurs when electrons and holes, which are moving in opposite directions in solar cell 100, combine with each other. Each time an electron-hole pair recombines in solar cell 100, charge carriers are eliminated, thereby reducing the efficiency of solar cell 100. Recombination may occur in the bulk silicon of substrate 110 or on either surface 105, 106 of substrate 110. In the bulk, recombination is a function of the number of defects in the bulk silicon. On the surfaces 105, 106 of substrate 110, recombination is a function of how many dangling bonds, i.e., unterminated chemical bonds, are present on surfaces 105, 106. Dangling bonds are found on surfaces 105, 106 because the silicon lattice of substrate 110 ends at these surfaces. These unterminated chemical bonds act as defect traps, which are in the energy band gap of silicon, and therefore are sites for recombination of electron-hole pairs.
As noted above, one function of the dielectric passivation layer 104 is to minimize the carrier recombination at the surface of the emitter region(s) 102 or the base region 101 over which the dielectric passivation layer 104 is formed. It has been found that forming a negative charge in a dielectric passivation layer 104 disposed over a p-type doped region formed in a solar cell device can help repel the carriers moving through the solar cell, and thus reduce the carrier recombination and improve the efficiency of the solar cell device. While it is relatively easy to form a passivation layer that has a net positive charge using traditional plasma processing techniques, it is difficult to form a stable negatively charged passivation layer on the surface of a silicon substrate.
Thorough passivation of the surface of a solar cell greatly improves the efficiency of the solar cell by reducing surface recombination. In order to passivate a surface of solar cell 100, such as surface 105, a dielectric passivation layer 104 is typically formed thereon, thereby reducing the number of dangling bonds present on surface 105 by 3 or 4 orders of magnitude. For solar cell applications, dielectric passivation layer 104 is generally a silicon nitride (SiXNY or abbreviated SiN) layer, and the majority of dangling bonds are terminated with silicon (Si), nitrogen (N), or hydrogen (H) atoms. But because silicon nitride (SiN) is an amorphous material, a perfect match-up between the silicon lattice of emitter region 102 and the amorphous structure of dielectric passivation layer 104 cannot occur. Hence, the number dangling bonds remaining on surface 105 after the formation of dielectric passivation layer 104 is still enough to significantly reduce the efficiency of solar cell 100, requiring additional passivation of surface 105, such as hydrogen passivation. In the case of multi-crystalline silicon solar cells, hydrogen also helps to passivate the defect centers on the grain boundary.
During normal processing of the solar cell device the p-type boron doped regions found in the solar cell may form an oxide layer, such as boron silicate glass (BSG) layer that is hard to remove prior to forming the dielectric passivation layer 104. The BSG oxide layer may be formed over the back side of a p-type substrate base region 101, or, alternately, the BSG layer may be formed over a p-type emitter structure. However, it is generally important to remove the formed oxide layer and clean the substrate surface to prevent contamination of the solar cell substrate during subsequent processing and improve the passivating effect of the dielectric passivation layer that is later formed over the substrate surface.
It is also desirable to assure that the solar cell efficiently converts as much of the optical energy received by the sun into electrical energy as possible. However, since sunlight may be scattered, refracted, diffracted, or reflected fairly easily, several different techniques have been developed to enhance light trapping in the solar cells to improve conversion efficiency. For example, a surface texture may be provided to increase the surface roughness, thereby assisting the light to be trapped and confined in the solar cell. Conventional surface texturing processes often utilize aqueous alcohol related compounds as a chemical source for substrate surface treatment. However, alcohol related compounds are flammable, which are fire hazard and be in environmental safety concern, thereby requiring special safety measures during processing. Also, alcohols evaporate at the temperatures needed to assure that the chemical activity of the etchants in the texturing solution is in an optimum range to effectively perform the texturing process. Evaporation of the alcohol components from the texturing bath thus leads to an unstable texturing bath composition when the processes are run at these elevated temperatures.
Therefore, there is a need for an improved method of cleaning a substrate prior to depositing a passivation layer, an improved method of forming a desirable charge at the surface of the solar cell device to minimize surface recombination of the charge carriers, and there is a need for a method to form a desirable surface texture on a surface of a solar cell to improve the formed cell's ability to trap incident light.